A pump device has been conventionally known which is provided with an impeller and a DC brushless motor (motor) for rotating the impeller (see, for example, Japanese Patent Laid-Open No. 2007-120367). In the pump device described in this Patent Literature, a flow rate of the pump device is increased and decreased depending on increase and decrease of a rotational speed of the motor. Further, in the pump device, a power source voltage change circuit, a PWM circuit and a drive circuit are disposed between a control circuit for controlling a flow rate of the pump device and the motor. The power source voltage change circuit is a circuit which increases and decreases an output voltage of a motor drive power source based on an output signal which is outputted from the control circuit depending on a target flow rate of the pump device. The PWM circuit is a circuit which generates a PWM signal whose duty ratio is varied based on the output signal from the control circuit and the drive circuit is a switching circuit which performs on/off operations of an output voltage from the power source voltage change circuit according to the PWM signal.
Further, conventionally, a closed loop control and an open loop control have been known as a control method for a DC brushless motor. In the closed loop control, the motor is controlled while a measured result of an actual rotating speed of the motor is fed back and, in the open loop control, the motor is controlled under a condition that a measured result of an actual rotating speed of the motor is not fed back. A rotational speed of the motor is controlled with a high degree of accuracy in a closed loop control. However, in the pump device described in the above-mentioned Patent Literature, the motor is controlled in an open loop control in order to simplify the control of the pump device.
However, when a motor is controlled in an open loop control like the pump device described in the above-mentioned Patent Literature, a rotational speed of the motor is varied due to variations of circuit elements which structure a circuit for driving the motor and, as a result, a flow rate of the pump device is varied. Further, when a motor is controlled in an open loop control, a rotational speed of the motor is varied due to variation of a pressure loss depending on conditions of piping connected to the pump device, variation of a voltage of the motor drive power source, variation of a motor load and the like and, as a result, a flow rate of the pump device is varied.
Especially, a PWM duty ratio is lowered at the time of a low flow rate when a flow rate of the pump device is small and thus a rotational speed of the motor is lowered and its motor torque becomes small. Therefore, at the time of a low flow rate, the flow rate is easily affected by a load variation and the like and, as a result, a variation rate of an actual flow rate with respect to a target flow rate of the pump device becomes large.